A miner who somehow gains control of a majority of the network’s mining power can defraud other users by sending them payments and then creating an alternative version of the blockchain in which the payments never happened. This new version is called a fork. The attacker, who controls most of the mining power, can make the fork the authoritative version of the chain and proceed to spend the same cryptocurrency again.

This sounds less like a hack and more like a consequence of “it’ll never happen” dismissal of possible abuses of the system as it’s designed.